1. Take battery voltage reading before starting the engine and record. Secondly, while engine at idle and after reaching normal operating temperature, turn on your a/c or heater on high, headlights, turn lights, fog lights, defogger, seat heater if equipped. While equipments are on take another voltage reading. If alternator is working you should get at least 14.5v. If you get less than 14v, your alternator is failing and must be replaced.
2. Another possibility is Parasitic drain. It means an electrical discharge or a device is drawing large amount of power while engine is off. It usually mean there's a short somewhere. For your vehicle, 40 - 50 mA discharge is about normal when engine is shut off. But when you have 400 - 500 mA discharge will probably drain your battery overnight or at the end of your work day.
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